Much of the electrical power in use worldwide is distributed as high voltage alternating current (AC) from the location at which it is generated to a location proximate to where is will be utilized. High voltage allows power transmission over long distance with reduced wiring cost and resistive losses. Voltage can then be reduced, usually in stages, to lower voltage suitable for intended loads. This power distribution system is often referred to as the “power grid” or, simply, “grid”.
However, most electrical and electronic devices operate as a substantially fixed voltage, referred to as direct current (DC) and are often arranged to be temporarily connected to the grid or disconnected therefrom during which time they are operated from batteries that may be recharged from the power distributed on the grid. Accordingly, such devices, referred to as “offline”, require power conversion from AC to DC power and often the DC power is regulated at a much lower voltage than the voltage available from a connection to the grid. (More specifically, “offline power supply” is defined as a power supply in which the line voltage is rectified and filtered without using a line frequency isolation transformer, which does not preclude inclusion of a high frequency isolation transformer.) Devices that perform such conversion and possibly voltage regulation are often referred to as “adapters” and may be integrated with the electrical device for which they supply power or constructed as a separate structure with wires and fixtures for connection to both the grid and the electrical device.
So-called flyback converters (essentially a buck-boost topology DC—DC converter including a transformer for isolation and so-called because the energy transferred to the secondary side of the transformer is reflected back to the primary side when the primary side is “off” or non-conducting) are widely used in offline applications. After rectification to obtain a DC voltage input power source and switching to obtain a chopped DC waveform for input to the transformer, a passive diode is a simple expedient for providing DC conversion on the secondary side of the transformer but suffers from high conduction losses due to both a forward voltage drop and resistive losses at significant conduction current. Such conduction losses can be greatly reduced by using a synchronous rectifier that is essentially a switch that is controlled to conduct during selected periods when voltage is available to be conducted as DC. For example, a power MOSFET rated for 100 volt operation is considered to be a good choice for such applications and can reduce conduction losses by about 80% compared with a diode.
However, some difficulties are presented in precisely controlling devices which are practical for synchronous rectifier (SR) applications, particularly in adapters such as those described above. In such adapters light weight and low volume are particularly desirable since the devices to which they supply power are also small and typically of relatively light weight to be conveniently portable or movable. Accordingly, it is desirable to operate the adapter at high frequency to minimize the required sizes of components therein such as the transformer and filter capacitors and required power factor correction (PFC) and electromagnetic interference (EMI) filters. Unfortunately, the parasitic inductance including leakage inductance of the transformer, printed circuit board (PCB) layout and the primary and secondary switches package parasitic inductance resonates with parasitic capacitance of the primary side switch when the primary side switch is off and while the oscillation can be damped relatively quickly, it cannot be damped in the short off-period at a suitably high frequency to provide for reduction of component size, typically about 1 MHZ. This same oscillation exists in the secondary side SR circuit and makes SR driving difficult; the oscillation often shortening the SR conduction period and compromising a large part of the loss reduction potentially derived from the use of an SR.